A well-known mass-analyzing method for identifying a substance having a large molecular weight and for analyzing its structure is an MS/MS analysis (or tandem analysis). FIG. 14 is a schematic configuration diagram of a general MS/MS mass spectrometer disclosed in Patent Documents 1 and 2 and other documents.
In this MS/MS mass spectrometer, three-stage quadrupole electrodes 12, 13, and 15 each composed of four rod electrodes are provided, inside the analysis chamber 10 which is vacuum-evacuated, between an ion source 11 for ionizing a sample to be analyzed and a detector 16 for detecting an ion and providing a detection signal in accordance with the amount of ions. A voltage±(U1+V1·cos·ωt) is applied to the first-stage quadrupole electrodes 12, in which a direct current (DC) U1 and a radio-frequency (RF) voltage V1·cos ωt are synthesized. Due to the effect of the electric field generated by this application, only a target ion having a specific mass-to-charge ratio m/z is selected as a precursor ion from among a variety of ions generated in the ion source 11 and passes through the first-stage quadrupole electrodes 12.
The second-stage quadrupole electrodes 13 are placed in the tightly sealed collision cell 14, and Ar gas for example as a CID gas is introduced into the collision cell 14. The precursor ion sent into the second-stage quadrupole electrodes 13 from the first-stage quadrupole electrodes 12 collides with the Ar gas inside the collision cell 14 and is dissociated by the collision-induced dissociation to produce a product ion. Since this dissociation has a variety of modes, two or more kinds of product ions with different mass-to-charge ratios are generally produced from one kind of precursor ion, and these product ions exit from the collision cell 14 and are introduced into the third-stage quadrupole electrodes 15. Since not every precursor ion is dissociated, some non-dissociated precursor ions may be directly sent into the third-stage quadrupole electrodes 15.
To the third-stage quadrupole electrodes 15, a voltage±(U3+V3·cos ωt) is applied in which a direct current (DC) U3 and a radio-frequency (RF) voltage V3·cos ωt are synthesized. Due to the effect of the electric field generated by this application, only a product ion having a specific mass-to-charge ratio is selected, passes through the third-stage quadrupole electrodes 15, and reaches the detector 16. The DC U3 and RF voltage V3·cos ωt which are applied to the third-stage quadrupole electrodes 15 are appropriately changed, so that the mass-to-charge ratio of an ion capable of passing the third-stage quadrupole electrodes 15 is scanned to obtain the mass spectrum of the product ions generated by the dissociation of the target ion.
In a conventional and general MS/MS mass spectrometer, the dimension of the collision cell 14 along the ion optical axis C which is the central axis of the ion stream is set to be approximately 150 through 200 mm. In addition, the supply of the CID gas is controlled so that the gas pressure in the collision cell 14 is a few mTorr. When, under such conditions, ions travel a radio-frequency electric field in the atmosphere of such a comparatively high gas pressure, the kinetic energy of the ions attenuates due to collisions with the gas, thereby the ions slow down. Since, in the collision cell 14 of the aforementioned conventional MS/MS mass spectrometer, the area where the ion are decelerated is long, the delay of the ions becomes significant, and some ions may even halt.
In the case where an MS/MS mass spectrometer is used as a detector of a chromatograph such as a liquid chromatograph for example, it is necessary to repeatedly perform an analysis at predetermined time intervals. If the delay of the ions is significant as previously described, ions that should normally pass through the third-stage quadrupole electrodes 15 may not be able to pass through it, which deteriorates the detection sensitivity. In addition, ions remaining in the collision cell 14 may come out at a timing when no ion should appear, which creates a ghost peak. Moreover, since it takes a longer time for an ion to reach the detector 16, the time interval of the repeated analysis needs to be determined taking such a situation into account, which may bring about a detection loss in a multi-component analysis.
In order to avoid such problems as previously described, conventionally and generally, a direct current (DC) electric field having a potential gradient in the direction of an ion passage is formed in the collision cell 14, so that an ion is accelerated by the effect of the DC electric field.
Patent Document 3 discloses a mass spectrometer in which an electric field having a potential gradient in the direction of the ion optical axis is formed to accelerate ions by applying a DC voltage to a radio-frequency ion guide inclined to the ion optical axis or by applying a different DC voltage to each of the rods dividedly placed in the direction of the ion optical axis, so that ions are accelerated. Patent Document 4 discloses a mass spectrometer in which ions are accelerated by successively applying pulse voltages to the aperture electrodes of a radio-frequency ion guide composed of about one hundred aperture plates arranged in the direction of the ion optical axis.
However, when the rod electrodes of a radio-frequency ion guide are inclined or deformed, or when an auxiliary electrode is used in order to form a DC electric field having a potential gradient in the direction of the ion optical axis, the radio-frequency electric field adequately designed for converging ions may be disturbed, and the ion transmission efficiency may be deteriorated. On the other hand, the mass spectrometer having the structure according to Patent Document 4 is difficult to control due to its complex structure and necessity to appropriately control the pulse voltages for accelerating ions in accordance with each mass-to-charge ratio.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. H07-201304    [Patent Document 2] Japanese Unexamined Patent Application Publication No. H08-124519    [Patent Document 3] U.S. Pat. No. 5,847,386    [Patent Document 4] U.S. Pat. No. 6,812,453